Walmart has the 16.5 Firestones, not too bad a price.
Firestone Transforce HT Tire 9.50R16.5/10
http://www.walmart.com/ip/Firestone-Transforce-HT-Tire-9-50R16-5-10/17790921
16.5's are somewhat obsolete but there are still many sources around.
TIRE RACK...
I have a 73 C30 dually that has the original 7.50x16 LR D bias tires and wheels on it. I have been looking in wrecking yards for tubeless rims to install on it. All the used GM rims I find have 1/2 inch stud holes. The studs on my 73 are 9/16 inch. Did this change with the 76 model year...